The annual world production of sewage sludge coming from the treatment of domestic waste water can be estimated to be near 50 million metric tons on a dry basis. The elimination methods most frequently used for the disposal of these sludges are incineration, sanitary burial and agricultural spreading. The cost of treatment and disposal of the sludges by these conventional elimination methods varies between $250.00 and $725.00 Canadian/metric ton. On this basis, the world market for the treatment of sewage sludge can be estimated at approximately 15 billion dollars annually.
Incineration and burial are the conventional methods of sludge disposal. However, increase of the volume of the sludge to be disposed of and the scarcity of dump sites as well as increasing social opposition to these methods of disposal have a tendency to favor methods of utilization, specifically, as agricultural or forest fertilizer. The agricultural utilization of sludge, the option preferred by governmental authorities, is practiced more and more all over the world. In the United States, in 1976, this method was used for the disposal of 26% of the sewage sludge produced by municipalities. In 1990, it exceeded 33% of the total volume of sludge produced. In Europe in general, approximately 37% of the sludge is used in agriculture. In the United Kingdom, more than 51% of the sludge produced is used for this purpose, while near 40% is disposed of in this way in Japan. In Canada, 29% of the total estimated volume of municipal sludge is deposited on agricultural soil.
However, the presence of pathogenic microorganisms and high concentrations of toxic metals in the sludge constitute an obstacle which significantly restricts the above practice. It is well known that conventional methods of sludge treatment, such as aerobic or anaerobic stabilization, are ineffective in removing toxic metals and not very effective in the destruction of pathogenic microorganisms. In North America and in Europe, more than 50% of the sludges produced by municipal water treatment plants according to a conventional digestion method contain concentrations of heavy metals which exceed the recommended standards for agricultural spreading, which thus makes them potentially toxic. It is well known that copper, nickel and zinc are phytotoxic and that high concentrations of these in the soil may greatly affect the harvest yield. Accumulation of metals in the plants as a result of spreading of sewage sludge was demonstrated for antimony, arsenic, cadmium, chromium, copper, iron, mercury, molybdenum, nickel, lead, selenium and zinc. Metals may also be found in the food chain by adhesion to the surface of plants, resulting from the application of sewage sludge on the soil.
The presence of heavy metals in the edible part of plants thus may prove to be a risk to human and animal health. For example, cadmium is an element particularly feared because symptoms of its phytotoxicity appear at concentrations which are near 10 times higher than that where the zootoxicity appears. In humans and in animals, excessive absorption of cadmium causes its accumulation in the kidneys and the liver, thus producing histological and functional damage. The biological effects of cadmium also include interference with fundamental enzyme systems, such as oxidative phosphorylase, by blocking of thiol groups, as well as interference with the synthesis of nucleic acids. Cadmium is also supposed to have some cardiotoxic properties. Lead is another element which has a zootoxicity potential higher than that of the phytotoxicity. Although the accumulation potential of this element in the food chain is low, toxic effects have been reported in bovines that were feeding on soil amended with sludge highly polluted with lead. Environmental exposure to low lead contents is associated with various metabolic disorders and neuropsychological deficiencies in humans, such as a damaging effect on the metabolism of red blood cells; perturbation of calcium homeostasis in hepatocytes, bone cells and brain cells; and neurological damage. Various studies also evidence the harmful effect of lead in human arterial hypertension.
Generally speaking, toxic metals such as Al, As, Sb, Be, Bi, Cd, Hg, Cr, Co, Mn, Ni, Pb, Ti, V, Se and Zn could damage human reproduction or be the initiator or promoter of certain cancers, by acting as inhibitor in the biosynthesis of DNA or RNA or even as a mutagenic agent. Results of numerous studies on the risks associated with the application of sludge containing heavy metals in agricultural soil have been published during the last few years.
The interest in reducing the toxic metal concentration in sewage sludge is a well-known fact today. Two types of intervention are envisaged in order to do this, either removal of the metals during waste water treatment and/or control at the source of industrial wastes. During the last few years, various techniques of extraction of metals have been examined, but, until now, no method was considered to be competitive on an economic level with the conventional techniques of sewage sludge disposal. Regarding reduction at the source, although desirable, this approach is not only costly, but also involves uncertain results because it is difficult to delineate the diffuse sources of pollution, which contribute to the enrichment of the sludge in toxic metals. As a matter of fact, several studies show that a large portion of the metals found in the sludge come from residences and urban surface waters.
Since 1975, several techniques have been examined for the removal of heavy metals from sewage sludge, but, so far, no economical and effective methods seem to have been installed on a commercial basis. Numerous attempts of chemical dissolution of metals have been studied, such as chlorination, ion-exchange, use of chelating agents such as ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic acid (NTA), and thermophilic self-heating aerobic digestion (TSAD), coupled with acidification with hydrochloric acid. The high operating costs, certain operational difficulties and sometimes unsatisfactory yields of metal leaching prevented the emergence of these techniques.
The addition of different organic acids (CH3COOH) and inorganic acids (H2SO4, HCl, HNO3) to the sludge is a technique which has been considered most frequently by various searchers. However, utilization of organic acids permits only low yields of dissolution of the metals, while involving prohibitive costs.
The use of inorganic acids alone does not permit efficient dissolution of copper and lead, in spite of a considerable acidification of the sludge, i.e., to a pH of the order of pH 1.5. The solubility of metals in the sludge is affected mainly by the pH but also by other factors which are all also important and must be taken into consideration, such as the oxidation reduction potential of the medium, the concentration of the metals and ligands, e.g., the anions and uncharged molecules, and chemical equilibrium between the constituents. The dissolution of copper and lead in the sludge requires significant increase of the oxidation reduction potential, which cannot be obtained rapidly by chemical oxidation during the aeration of the sludge. The large quantities of acid necessary to dissolve the metals make these techniques not very attractive economically.
The combined use of an acid and a strong oxidizing agent was also examined. Several searchers suggested the use of hydrochloric acid and hydrogen peroxide at a pH ranging from 1.0 to 1.5, which permits the achievement of better dissolution yields of the metals than by the addition of only an acid. However, the operating costs of this technique are high because the quantity of acid required to lower the pH to such a value is very large, without even considering that the fertilizing and nutrient elements of the sludge are then degraded or dissolved.
The method described in U.S. Pat. No. 5,051,191 comprises a very significant acidification of the sludge (pH 1.0 to 2.0), by the addition of sulfuric acid or hydrochloric acid, coupled with the addition of an oxidizing agent in the form of ferric salts (sulfate or chloride) to a concentration varying between 0.5 and 3.0 g of Fe3+/L (sulfate or chloride) and the addition of an agent that regenerates the oxidizing agent, such as hydrogen peroxide, sodium or calcium hypochlorite, compressed air, oxygen, ozone, sulfur dioxide, chlorine or chlorinated compounds. A period of treatment of 10 to 30 minutes is sufficient with this technology for adequate dissolution of the heavy metals. The decontamination line also includes a step of conditioning the sludge by flocculation with a cationic or anionic polymer, followed by dehydration of the sludge on a drum filter under vacuum, and washing the decontaminated sludge.
The need to add considerable quantities of acid and oxidizing agent, as well as the utilization of a regenerating agent, results in operating costs which greatly restrict the marketing of this technology, even if it offers good yields of sludge decontamination. As an example, the acidification of the sludge to a pH of 1.5 requires approximately 90% more sulfuric acid than acidification of it to a pH of 2.5. Such an increase in acid consumption almost doubles the cost required for the acid and thus decreases the attractiveness of the method. The washing of the sludge during the dehydration step on a drum filter under vacuum is also an additional necessity, which involves an increase in the cost of sludge treatment.
It should also be considered that pronounced acidification of the sludge involves excessive dissolution of nutrient elements (nitrogen and phosphorus) in the sludge, which are then found inevitably in the final effluent after dehydration of the leached sludge. This phenomenon also contributes significantly to reducing the agronomic value of the sludge decontaminated in this way. For example, comparative tests on physicochemical treatment sludges from waste water showed that acidification of the sludge to a pH of 1.5 resulted in a loss of 44% of total phosphorus, while under the same experimental conditions, but at a pH of 2.5 and using the same sludge, the loss of this element is only 6%.
Another disadvantage of the method of U.S. Pat. No. 5,051,191 is the great difficulty of adequately flocculating a sludge leached at a pH below 2.0. Under these conditions of acidity, the flocks obtained by conditioning with organic polymers are of low dimension and are fragile. Dehydration of sludges flocculated in this way is very difficult with the standard equipment employed for the dehydration of sludges (filters with pressurized belts, rotary disk filters, centrifuges). As a matter of fact, the patent suggests the use of a drum filter under vacuum for the dehydration of the sludge treated by its method. Now, this type of equipment is not used for the dehydration of sewage sludges from municipal waste waters. Washing of the sludge during the dehydration step on a drum filter under vacuum, a technique which seems to be poorly adapted to the dehydration of sewage sludge, also constitutes an additional requirement, which results in an additional increase in the sludge treatment costs.
Attempts have also been made to separate metals by centrifuging. Two successive centrifuging steps permit concentration of the metals in a deposit. The concentrations of metal found in the deposit are between 60 and 73% for cadmium, nickel, chromium, copper and zinc, but this method does not permit extraction of the lead. This technique presents problems from the point of view of recovery and utilization of solids, because the sludge in which the metal contents are reduced represents only 23% of the total sludge volume.
Extraction of metals using a magnetic method and ion-exchange method was also studied. The metals in the sludge are captured by an ion-exchange resin which is regenerated subsequently in an acidic medium. The yields of removal of the metals by this technological approach in artificially contaminated sludges are 57% for copper, 66% for zinc and 86% for cadmium. However, the economical feasibility of this approach does not seem to have been demonstrated for application on an industrial scale.
A new chemical process including dissolution of metals followed by chelation on a solid support was developed for the decontamination of soils (see Mourato D. and D. D. Lang (1994) The Toronto harbour commissioners soil recycling demonstration project, summary of operations and test results. Final report. The Toronto harbour commissioners and Zenon Environmental inc., 46 pages). The authors of this report on the development of this process claim to have been capable of decontaminating previously digested sewage sludges. The details given in the report do not permit evaluation of the technical and economical feasibility of the application of this technology for the decontamination of sewage sludge. On the other hand, obligation to treat digested sludges decreases its applicability and certainly increases the overall cost of treatment of the sludge, making the process economically less interesting.
The utilization of organic acids in the methods of sludge treatment or of large quantities of inorganic acids necessary to lower the pH results in a significant increase in the operating costs associated with the chemical products. Thus, the high cost of these methods, the inadequateness of the equipment required for the sludge treatment stations, the insufficient yield of removal of toxic metals and the loss of fertilizing values are the main obstacles for marketing the sludge decontamination technologies which were proposed until now.
Considering the various technical and economical constraints encountered with the chemical processes of removing the metals associated with municipal sludge, interest in developing a biological extraction method appeared. During these last years, some bioleaching studies have been carried out.
A technology developed by INRS consists in a method of bioleaching of heavy metals using ferrous sulfate. The method is used for the decontamination of sewage sludge which previously underwent a microbiological stabilization step by aerobic or anaerobic digestion. The reaction time in the bioreactor varies between 1 and 2 days depending on the mode of operation and on the specific sludge to be treated. Addition of ferrous sulfate is necessary as an energy source substrate. The conditions of acidity thus created and the increase of the oxidizing conditions in the medium during the oxidation of ferrous ions to ferric ions permit significant dissolution of the toxic metals found in the sludge.
The above-mentioned biological processes permit to circumvent a good part of the problems inherent to the chemical processes, thus reducing the costs attributable to chemical products appreciably. However, the treatment times remain long (1.5 to 12 days) in comparison to the chemical processes (0.02 to 0.25 day), which result in an increase of the initial investment, such as the purchase price of the bioreactors and the cost of operation, particularly in respect of electricity.
Thus, it would be advantageous to develop a process combining the advantages of chemical and biological leaching processes, which would provide a low-cost process involving a relatively short treatment time and not requiring the purchase of expensive equipment for performing it.